Medical apparatus for the measurement of respiratory flow independent of gaseous composition

ABSTRACT

Respiratory flow rate is measured independent of gas type by establishing a supplementary flow component of the respiratory medium via a supplementary sensor modeled according to flow rate ratio after the primary respiratory flow sensor such that the sensors are subject to respective corresponding errors in dependence on the same parameter of any of the respective respiratory media. The respiratory flow signal is continuously corrected in dependency on the currently existent composition of the respiratory medium by forming the quotient of the respiratory flow signal and the supplementary signal from the supplementary sensor. In a second disclosed embodiment, the supplementary flow component is coupled to the primary respiratory flow sensor so that similitude is automatically provided. Either embodiment may be utilized to provide an inverse of the supplementary signal as a measure of a composition-dependent parameter such as viscosity, density or temperature coefficient of the respiratory gases.

CROSS REFERENCE TO RELATED APPLICATION

A continuing application is being filed on or about June 30, 1978,directed particularly to broader aspects of the embodiment of FIG. 1 ofthe present application.

BACKGROUND OF THE INVENTION

This invention relates to medical apparatus for the measurement,independent of gas type, of respiratory flow, having a breathing tubeand a flow measurement sensor accommodated therein, for example aFleisch pneumotachograph, a flow area restricting differential pressuretype flowmeter, a thermal flowmeter, or the like.

Known devices for respiratory flow measurement operate, for example,with Fleisch pneumotachographs as respiratory flow receptors, suchreceptors being based on the principle that respiratory flow issubjected in the receptor to an effective flow resistance, the flowpressure being measured at two points, that is before and after the flowresistance. The pressure difference is proportional to the volume flowrate V, with a proportionality factor containing geometrical apparatusconstants and, further, parameters dependent on the gas type of therespiratory medium, for example the viscosity μ. As a generalization,the signal output (S) from the usual flow measurement sensors can beexpressed as a product of two functions, a flow function f(V) and aparameter function g(m_(i)), that is: S = f(V) · g(m_(i)), where m_(i)for example signifies one of the gas parameters: viscosity μ, density ρand temperature conductivity λ. Other parameters are possible.

Because of the effect of the gas parameters, the range of application ofthe subject type of measurement apparatus has been limited. To be sure,the respiratory flow receptors can, by means of calibration withstandard flows of a gas or also of a gas mixture of known composition,for example room air, be used for the measurement of the flow of thesame respiratory gases with known composition. Such receptors are lesssuitable for the respiratory flow measurement of gases and gas mixturesof arbitrary or variable composition. The gas parameters must then, ifnecessary, be separately determined in special devices or be calculatedfor the respiratory gas mixture after a quantitative gas analysis, whichcan be very expensive. Thereafter, the result of the flow measurementmust be corrected in accordance with the ascertained gas parameters. Forthe measurement of respiratory flow in connection with the evaluation oflung function by known techniques, such procedures are too expensive,considering the required exactness and speed of the measurement. Itwould indeed be desirable to carry out respiratory flow measurementswith various respiratory gases at variable temperatures and partialpressures.

SUMMARY OF THE INVENTION

The invention thus has the underlying objective of eliminating thisdisadvantage. Medical apparatus of the type referred to at the outset isto be disclosed, which can be used for the measurement of respiratoryflow with gases and gas mixtures of arbitrary or variable compositionand temperatures, without the above named special devices and proceduresbeing necessary to that end.

This objective is inventively accomplished in that, for the continuouscorrection of the measured values as a function of the composition andcharacteristics of the respiratory gases, the breathing tube has anauxiliary means comprising a pump, preferably an alternating pressurepump, which extracts a sample from the respiratory flow, and, ifnecessary, returns it thereto. The auxiliary means extracts such samplefrom the fluid medium of the breathing tube via a supplementarymeasurement sensor dependent on the same gas parameters and themeasurement signal produced by such supplementary measurement sensorpreferably has the same functional relationship to flow rate as themesurement signal which is produced by the primary flow measurementsensor in the breathing tube. Further, means are provided for theformation of a respiratory flow indication which is a function of thequotient of the measurement signals obtained on the basis of therespiratory flow and of the supplementary flow produced by the auxiliarymeans. In such medical apparatus it is advantageous to make thesupplemental measurement sensor smaller in accordance with the ratio ofthe supplementary sample flow to the respiratory flow, so that in thesupplementary measurement sensor the same laws of fluid mechanics arevalid as apply to the primary flow measurement sensor which senses theentire respiratory flow. The objective is further inventivelyaccomplished in that, for the correction of the measurement values as afunction of the composition and the characteristics of the respiratorygases, the breathing tube has auxiliary means comprising a pump,preferably an alternating pressure pump, which loads the respiratoryflow with a defined supplementary flow component having a frequencywhich is capable of being differentiated from the breathing frequency,the supplementary flow component together with the respiratory flowbeing conveyed via the flow measurement sensor, and means being providedfor the separation of, and for quotient formation as a function of, thelow-frequency and high-frequency components of the measurement signalsobtained on the basis of the combined presence of the respiratory flowand the supplemental flow component. It is advantageous in each instanceif the supplementary flow impressed by the alternating pressure pump isa sinusoidal pulsation whose frequency is clearly above the breathingfrequency.

Other objects, features and advantages of the present invention will beapparent from the following detailed description taken in connectionwith the accompanying sheet of drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of a first exemplary embodiment ofmedical apparatus for respiratory flow measurement where the fluidmedium within the apparatus may be made up of any arbitrary gas type,and where provision is made for the continuous correction of the effectof the viscosity of such fluid medium;

FIG. 2 is a schematic representation of a second exemplary embodiment;and

FIG. 3 is a schematic representation of a suitable flow orifice for thecontinuous correction of the effect of the density of the respiratorymedium during the course of respiratory flow measurement.

DETAILED DESCRIPTION

In FIG. 1 a breathing tube is designated with the reference numeral 1.This breathing tube has, at its end located opposite the mouthpiece (notshown), a respiratory flow receptor 2 which operates according to theprinciple of the Fleisch pneumotachograph. The tapped differentialpressure Δp which is generated as a function of flow in the breathingtube is converted by means of a transducer 3 into an analogouselectrical signal (S). This signal (S) is proportional to the product ofthe viscosity μ and the volume flow (V) to be measured. On the breathingtube there is additionally located an auxiliary duct 4 with analternating pressure pump 5 (a diaphragm pump, for example,corresponding in its manner of actuation to a loudspeaker diaphragm, ora piston pump) and with a further respiratory flow receptor 6 which alsooperates according to the principle of the Fleisch pneumotachograph.With the alternating pressure pump 5 a small sample of known magnitudeis periodically extracted from the fluid medium flowing in the breathingtube 1, the pump 5 preferably being operated according to a sinusoidalfunction with known frequency and producing at the receptor 6 a definedvolume flow (V₁), the pump extracting such supplementary flow componentfrom the respiratory flow (V) to be measured, the sample being extractedvia the receptor 6 in one-half cycle of the pump operation and beingreturned again into the respiratory flow on the next half cycle. Thereceptor 6 is made smaller in accordance with the small sample (V₁) tobe extracted from the gas flow (V), in comparison with the respiratoryflow receptor 2, according to the laws of similitude of fluid mechanics.The measurement differential pressure signal Δp₁ is converted in thepressure transducer 7 into an analogous electrical signal (S₁) and, inthe rectifier 8 with following low-pass filter 9, this analog signal isrectified and smoothed. The signal supplied by the low-pass filter 9 isproportional to the product of the known volume flow (V₁) and theviscosity μ. In the dividing component 10 which receives the signals (S)and (S₁), the quotient of the signals S/S₁ is formed. By the quotientformation S/S₁ the viscosity μ is eliminated and, since (V₁) is known, asignal S' dependent only on the volume flow V to be measured isobtained.

In FIG. 2 the reference numeral 11 designates a flow measurementconduit, constructed as a breathing tube, with a test subject'smouthpiece 12 at one end for producing a respiratory flow V. Thebreathing tube 11 in turn has an auxiliary duct 13 with an alternatingpressure pump 14 which loads the breath flow V with a definedsupplementary pulsation V₁ which is at a higher frequency than thebreathing frequency. Between the measurement tube 11 and the mouthpiece12 there is located a longer hose 15 (for example with a length of ninehundred millimeters, 900 mm, and with an inside diameter of 12millimeters, 12 mm) which hose represents an approximately purelyinductive, sufficiently large flow resistance (for example, 1.5millibars per liter per second, 1.5 mbar/l/s at a respiratory volumeflow of 0.5 liter per second, V = 0.5 l/s) and guarantees that thesupplementary pulsation has a defined substantial effect on the flowmeasurement sensor 16. Further, the hose 15 prevents reactions of theimpressed alternating supplementary flow component on the breathingpattern. At the open end of the breathing tube is situated therespiratory flow receptor 16 which is according to the principle of theFleisch pneumotachograph. The differential pressure signal from receptor16 is effected by a combination of the low-frequency respiratory flow Vto be measured and the supplementary flow component which contains theknown high-frequency volume flow pulsation V₁. The analogous electricalsignal S_(o) formed at the output of the pressure transducer 17 thuscontains low-frequency and high-frequency components. By a parallelconnection of a low-pass filter 18 for transmitting the respiratoryfrequency and blocking the high-frequency component, and of a band-passfilter 19 tuned to the higher-frequency pulsation, a separation of thelow-frequency and high-frequency components S and S₁ is achieved. Thehigher-frequency component S₁ is rectified and smoothed by means ofrectifier 20 and subsequent low-pass filter 21. The low-frequency signalS is proportional to the product of viscosity μ and the volume flow V tobe measured, while the smoothed signal S₁ is only dependent on theviscosity, since the supplementary volume flow component V₁ is known andtherefore only enters into the calculation in the form of aproportionality factor. By means of the quotient formation S/S₁ in thedividing component 22, a signal S' which is dependent only on the volumeflow V to be measured is then obtained. The proportionality factor isdependent on the magnitude of the pulsation of the alternating pressurepump 14 and can be calibrated into the apparatus to obtain a directreading of respiratory flow V in liters per second.

Inventive devices for the respiratory flow measurement corrected for theeffect of density ρ and the temperature conductivity λ are constructedin a correspondingly equivalent manner. As measuring sensors, then, floworifices or Venturi tubes or, thermal flowmeters such as electricallyenergized temperature sensitive resistance elements, or pyroelectricelements are used. The orifice element 24, FIG. 3, is shown arranged inan auxiliary duct 23 which in one embodiment corresponds to theauxiliary duct 4 according to FIG. 1 and in another embodimentcorresponds to the auxiliary duct 13 according to FIG. 2. For densitycompensation, the orifice element 24 must fulfill specific conditions asto its dimensioning in order to deliver a signal directly proportionalto the density to the transducer 7 of FIG. 1 or the transducer 17 ofFIG. 2. Thus applying FIG. 3 to the embodiment of FIG. 1, thesupplementary flow component V₁ produces a pressure differential acrossthe orifice element 24, Δp₁ which is directly proportional to thedensity and is applied to the pressure transducer 7. In the embodimentof FIG. 3 according to FIG. 2, the signal V₁ transmitted from the pump14 via the orifice element 24 to the receptor 16 (or preferably acorresponding orifice) is used to generate a superimposed pressuredifferential signal Δp₁ which is directly proportional to the density ofthe fluid medium. Investigations have shown that, with a diameter D ofthe pump auxiliary duct 23 of 12 millimeters (D = 12 mm), the orificeelement 24 with sharp edges and a diameter d of 2.2 millimeters (d = 2.2mm) fulfills this requirement. The frequency of the alternating pressurepump 5 or 14 is at 15 hertz (15 Hz); the measuring time constant is at200 milliseconds (200 ms) in each case.

The measurement signal magnitudes S_(i) such as the signals S and S₁referred to in connection with the exemplary embodiments of FIGS. 1 and2, need not necessarily be directly proportional to the product of theinfluencing magnitudes or factors, such as for example, the viscosity μ,density ρ and the temperature conductivity λ, and the volume flow V.More complicated functional dependencies corresponding to a signalstructure S = g(m_(i)) · f(V), where m_(i) signifies one of the possiblegas parameters, are possible, which then require, in accordance with theknown functions g(m_(i)) and f(V), a modification of the evaluation andcalculating components. If the flow measurement sensors and thesupplementary measurement sensors have different gas parameter functionsg₁ (m_(i)) and g₂ (m_(i)), then these gas parameter functions must firstbe converted into a common gas parameter function g(m_(i)) which maycorrespond to g₁ (m_(i)) or g₂ ( m_(i)).

Such devices are generally applicable in gas flow measurement, inparticular however in respiratory flow measurement with various testgases used in the lung function measurement technique, such as forexample argon (Ar), Nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂),nitrous oxide (N₂ O), helium (H_(e)), as well as water vapor (H₂ O) andthe mixtures thereof. The pulsation frequency of the alternatingpressure pump is always to be not greater than about fifteen hertz (f₁pump≦ 15 Hz) and, in particular, is to be adjusted to the gases or,respectively, gas mixtures which are to be subject to the measurementtechniques of the present invention. It must, however, be clearlyseparable technically from the breathing frequency. The achievablemeasurement accuracies lie near 2.5%. Further, with inventive devices ofthis type, measurements of other fluids, for example flowing liquids,can also be carried out. For this purpose, the correction of the flowmeasurements by the superimposition of an auxiliary or supplementaryflow component operates in the sense of a continuous calibration.

In addition to the described respiratory flow and gas flow measurementsit is also possible, with the inventive devices, to quickly andcontinuously determine the gas parameters themselves, whose effect waseliminated from the measurement signals in the continuous correction. Byconnecting an inverse-function-former after the low-pass filter 9 ofFIG. 1 or, respectively, after the low-pass filter 21 of FIG. 2, theviscosity μ can be directly read off. Density ρ and temperatureconductivity λ can be measured the same way using correspondingmeasurement sensors.

SUPPLEMENTAL DISCUSSION OF THE FEATURES OF THE ILLUSTRATED EMBODIMENTS

1. FIG. 1, and FIG. 3 as applied to duct 4 of FIG. 1, provide medicalapparatus for the measurement of respiratory flow rate V(t), independentof the type of respiratory gas or gases making up the fluid mediumwithin the apparatus. The flow measurement sensor 2 has been illustratedas a "Fleisch nozzle" in FIG. 1. (See, for example, the article "DerPneumatoachograph" from "Neue Methoden zum Studium des Gasaustauches undder Lungenfunction" by A. Fleisch, VEB Thieme Leipzig, 1956). The flowmeasurement sensor 2 may also operate on the principle of throttled flowor on the principle of heat transfer fron an electrically-energizedhot-wire flowmeter (that is, a temperature-sensitive rsistance elementwhich thus may control electric current flow therethrough according toambient gas flow rate), or the like. An important feature of suchembodiments is characterized in that, for the continuous correction ofthe respiratory flow rate measurement signal from sensor 2, thebreathing tube 1 has an axuiliary duct 4 or 23 connecting therewith witha pump, preferably an alternating pressure pump (such as 5), whichextracts a sample from the respiratory flow at a suplementary flow rateV₁ (t), and, if necessary, returns the sample thereto, and conveys thesample via a supplementary measurement sensor (such as receptor 6 ororifice element 24). The supplementary sensor is dependent on the samegas parameter (m₁) as affects the accuracy of the flow rate indicationfrom the primary measurement sensor (such as receptor 2). The signal(such as Δ p₁) from the supplementary mesurement sensor (6 or 24)preferably has the same functional relationship (operates on the sameflowmetering principle) to flow rate V₁ (t) as the signal (such as Δ p)from the primary measurement sensor 2 has to its associated flow rateV(t). The supplementary means for continuously correcting therespiratory flow signal (S) includes further means (such as analogdivider component 10) for forming the quotient (S/S₁) of the signals (Sand S₁) obtained from the sensor 2 on the basis of respiratory flow V(t)and from the sensor 6 or 24 on the basis of supplementary flow componentV₁ as generated by the pump (such as 5).

2. A further characterizing feature of such embodiments of FIGS. 1 and 3resides in that the sample extracted by the pump (such as pump 5 and viaorifice 24 for the example of FIG. 3) producesa supplementary flowcomponent V₁ which is small in comparison to the respiratory flowcomponent V, and the supplemental measurement sensor (such as receptor 6or 24) is made smaller in its significant physical dimensions accordingto the laws of dynamic similarity of fluid mechanics, in accordance withthe ratio of supplementary flow V₁ to respiratory flow V. For the casewhere the pump such as 5 produces a sinusoidal supplementary flowcomponent with a peak amplitude V₁ (peak) and the sensor 2 responds to amaximum amplitude of respiratory flow of V (max), the revelant ratio forpurposes of the laws of similitude of fluid mechanics may be taken as V₁(peak) / V (max). By way of example, the ratio V₁ (peak) / V (max) maybe less than one-tenth.

3. FIG. 2 and FIG. 3 as applied to duct 13 of FIG. 2 also illustratemedical apparatus for the sensing of respiratory flow rate V as acontinuous function of time, using a Fleisch pneumotachograph, a sensorbased on throttled flow (for example a Venturi), a thermal flowmeter(for example a hot-wire flowmeter), or the like. An important feature ofthese embodiments is characterized in that, for the continuouscorrection of the respiratory flow rate signal S(t) which is producedunder the control of sensor 16 as a function of the composition and thecharacteristics of the respiratory gas or gases making up the fluidmedium within the apparatus, the breathing tube 11 has auxiliary means(such as 13, 14; and also orifice 24 for FIG. 3) including a pump,preferably an alternating pressure pump (such as 14), which loads therespiratory flow V (t) with a supplementary flow V₁ having asupplementary flow component with a frequency which is capable of beingdifferentiated from the breathing frequency, the supplementary flowcomponent together with the respiratory flow V(t) being coupled with theflow measurement sensor 16, so that the supplementary signal S₁ (t) andthe respiratory flow signal S(t) are both dependent on a parameter (m₁)of a given fluid medium which affects the accuracy of the signal S(t) asa measure of respiratory flow rate V(t). By way of example, with theillustrated embodiments, the signal S(t) may deviate from the correctvalue because of differences in viscosity (for FIG. 2) or density (as toFIG. 3) for respective respiratory gas compositions such as argon,nitrogen, oxygen, carbon dioxide, nitrous oxide, helium, water vapor andthe mixtures thereof, by an amount substantially exceeding plus or minus2 5/10 percent. Supplementary measurement means (such as 18, 19 and 22)are present for separation of the composite signal S_(o) (t) intolow-frequency and high-frequency components S(t) and S₁ (t), where thesignal S_(o) (t) is obtained on the basis of the mixture orsuperimposition of the respiratory flow V(t) and the supplementary flowV₁ (t). The supplementary measurement means includes further means (suchas analog divider component 22) for the formation of the quotientS(t)/S₁ (t) as a continuous function of time, thereby to provide arespiratory flow indication S'(t) at the output of the further quotientformation means which is substantially continuously compensated for theparameter (m_(i)) of the fluid medium substantially independently of thetype of respiratory gas or gases making up the fluid medium.

4. With either the embodiment of FIG. 1 or FIG. 2 or either embodimentcombined with FIG. 3, a further feature is characterized in that thesupplementary flow V₁ (t) produced by the alternating pressure pump(such as 5 or 14) is the result of a sinusoidal pulsation at animpressed frequency lying clearly above the breathing frequency.Preferably such impressed frequency is substantially higher than thehighest anticipated breathing frequency (corresponding to therespiratory rate), and preferably such impressed frequency is betweenabout 5 hertz and about 15 hertz.

5. In an embodiment such as illustrated in FIG. 2, or FIG. 3 taken withFIG. 2, where a substantial amplitude of the supplementary flowcomponent V₁ is to be present in the respiratory flow tube (such asbreathing tube 11), a further feature is characterized in that betweenthe respiratory flow tube and the patient's mouthpiece 12, asufficiently large pneumatic flow resistance is arranged, preferably ahose line (such as 15) which is of substantial length and which presentsa substantial (e.g. reactive) flow resistance. The flow resistance ispreferably sufficiently large so that the supplementary flow componentV₁ has a substantial effect at the flow measurement sensor; and/or sothat such flow resistance prevents perceptible effects at the mouthpiece12 due to the supplementary flow component, such as might otherwisedisturb the patient or cause the patient to alter his breathing pattern.

6. In conjunction with any of the foregoing features, the apparatus isfurther characterized in that for the evaluation of the time-continuousmeasuring signals (such as Δp and Δp₁, FIG. 1), a measurement transducer(such as 3, 7, or 17) is provided for each such signal, so as to convertthe time-continuous measuring signals supplied by the flow measurementsensors (such as 2, 6, 16 and 24) into respective analogous (analog)electrical signals which control the respective time-continuous analogelectrical signals (such as S, S₁, FIG. 1; and S_(o), FIG. 2) suppliedto the supplementary measurement means (10, FIG. 1; and 18-22, FIG. 2.).In FIG. 1, the signal S₁ supplied to divider 10 is not identical to theoutput signal from transducer 7, but is nevertheless controlled thereby;the same terminology being applicable to an identical signal at theinput and output ends of a single conductor, or example.

7. A further feature applicable to FIG. 2, and FIG. 3 taken with FIG. 2,is characterized in that, for the separation of the low-frequency andhigh-frequency components S(t) and S₁ (t) of the composite electricalsignal S_(o) (t), electronic filter means are used, preferably aband-pass filter (such as 19) tuned to the pulsation frequency of thealternating pressure pump and a low-pass filter (such as 18) for thetransmission of the breathing frequency component of signal S_(o) (t).

8. As shown in FIGS. 1 and 2, the higher frequency component fromtransducers 7 to 17 is rectified by component 8 or 20 and smoothed in alow-pass filter component 9 or 21 to supply a time-continuous analogsignal S₁ (t) which is proportional to the peak amplitude of thesinusoidal signal V₁ supplied to the transducer 7 or 17, which has aconstant known value, and the signal g₁ (m_(i)) which is a function ofthe gas parameter(m_(i)) of the particular respiratory fluid medium.

9. With respect to any of the embodiments, the formation of the quotientS(t)/S₁ (t) is accomplished by electronic divider components (10, 22)such that the respiratory flow indication S' is substantiallycontinuously corrected with reference to any changes in the effect ofparameter (m_(i)). If the function g₁ (m_(i)) is not linearly related tothe function g(m_(i)) applying at receptor 2 in FIG. 1, or the similarembodiment as taught by FIG. 3, then g₁ (m_(i)) can be operated upon aspart of the output of low-pass component 9 to generate the signal S₁ (t)which is a function of the product of V₁ (peak) and g(m_(i)).

10. With respect to the foregoing feature No. 9, if need be electroniccalculating components may perform a conversion of the parameterfunction V₁ ·g₁ (m_(i)) of the supplemental measurement sensor 6 intothe function S₂ (t) which is linearly related to the parameter functiong(m_(i)) such that the respiratory flow indication S' = S/S₂ is formedby the quotient of V(t)·g(m_(i)) and k· g(m_(i)), where k is a constant,the inaccuracy introduced by the parameter function g(m_(i)), forexample the viscosity function g(μj), the density function g(ρj), and /or the temperature conductivity function g( λj) dependent on thecomposition of the respiratory medium (j), thus being essentiallyeliminated to give a measurement accuracy such as previously mentioned.

11. Medical apparatus is also envisioned for measuring parameters of thementioned respiratory gases and also of other fluids, for exampleflowing liquids, utilizing a flow measurement sensor or sensors as inFIGS. 1, 2, or 1 or 2 as modified by FIG. 3, for example using one ormore Fleisch pneumotochographs; one or more throttled flow devices (e.g.orifice plates or Venturi tubes); or one or more thermal flowmeters(e.g. hot-wire flowmeters); or the like. Such embodiments arecharacterized in that, for the rapid and continuous measurement anddisplay and/or recording of the parameters (m_(ij)) of fluid media (j₁,j₂, j₃, . . . . , etc.), for example viscosity μ, density ρ, andtemperature conductivity λ, which parameters are dependent on thecomposition of the fluid medium, there are provided supplementarymeasurement means coupled with the output of component 9 or 21, for theformation of the inverse function 1/S₁ (t) where S₁ (t) is a function ofthe known constant V₁ (peak) and g(m_(ij)), g (m_(ij)) being an inversefunction of (m_(ij)) since increased viscosity, density or temperatureconductivity, for example would result in a reduced signal to tranducer7 or 17.

12. From the method standpoint, a feature of FIG. 1 resides inextracting a sample from the respiratory fluid medium of such magnitudeand at such a rate that the respiratory flow is still properlymeasurable to provide a function S(t) but is subject to an inaccuracy inproportion to a parameter function g(m_(ij)) for respective fluidcomposition (j), and utilizng the extracted sample to generate a signalS₁ as a measure of the parameter function g₁ (m_(ij)) such that theparameter (m_(ij)) may itself be displayed or stored, or may be utilizedto correct the function S(t) for such inacuracy. By way of example, theextracted flow rate may be sensed by means of a supplementary sensormodeled by the principles of dynamic similarity after the primary sensorwhich generates signal S(t) to generate a parameter function g₁ (m_(ij))which bears a predetermined relationship to g(m_(ij)) for each of a setof fluid media compositions (j₁, j₂, j₃ , . . . , etc.) such that g₁(m_(ij)) can be converted to g(m_(ij)) with substantial accuracy for anymember of the set (ij). Preferably the value g₁ (m_(ij)) is sensed anewin each cycle of the supplementary flow component V₁ so that thecorrection function g(m_(ij)) is available for essentially continuouscorrection of the signal S (t) representing the measured respiratoryflow; that is the rate of correction of S (t) may be at least severaltimes the basic maximum respiratory rate of say 1 hertz, and preferablyat least five times such basic maximum rate, such rate ofsupplementation in each case being such that for practical purposes ofaccuracy of the respiratory flow rates the correction may be consideredto be continuous.

The foregoing features have been given by way of example, and without anintention to exhaustively list all important features; furthermore, thefeatures have been discussed having reference to the illustratedembodiments for the sake of ease of explanation and it will beunderstood that many equivalent means and steps of implementation willoccur to those skilled in the art from a consideration of thisdisclosure taken in its entirety.

While there have been disclosed exemplary embodiments representingpresently preferred practice of the claimed invention, it will beapparent that many modifications and variations may be effected withoutdeparting from the scope of the novel teachings and concepts of thepresent invention.

I claim as my invention:
 1. In medical apparatus for respiratory flowmeasurement, including a breathing tube for containing a fluid mediumsubject to respiratory flow varying at a breathing rate, and a flowmeasurement sensor accommodated in said breathing tube for coupling withthe respiratory flow of the fluid medium in said breathing tube tocontrol the supply of a respiratory flow signal as a function of suchrespiratory flow, said breathing tube having auxiliary means coupledwith the fluid medium in said breathing tube for producing asupplementary flow component, supplementary measurement means responsiveto the supplementary flow component to supply a supplementary signalwhich is dependent on a parameter of said fluid medium which alsoaffects the accuracy of said respiratory flow signal, and evaluationmeans operatively connected with said flow measurement sensor andresponsive to said respiratory flow signal and to said supplementarysignal for forming a quotient signal as a function of the quotient ofsaid respiratory flow signal and said supplementary signal to provide arespiratory flow indication compensated for said parameter of the fluidmedium substantially independently of the type of respiratory gas orgases making up said fluid medium, said auxiliary means comprising apump coupled with said fluid medium of said breathing tube forextracting a sample of said fluid medium from the breathing tube toproduce said supplementary flow component, said supplementarymeasurement means comprising a supplementary flow measurement sensorcoupled with said supplementary flow component to control the supply ofsaid supplementary signal, and said evaluation means being operativelyconnected with said flow measurement sensor in said breathing tube toreceive the respiratory flow signal controlled thereby and operativelyconnected with said supplementary flow measurement sensor for receivingsaid supplementary signal, and being operable for supplying saidrespiratory flow indication which is a function of the quotient of therespiratory flow signal and the supplementary signal.
 2. Medicalapparatus in accordance with claim 1, with said auxiliary meanscomprising an alternating pressure pump coupled with the fluid medium ofsaid breathing the tube via said supplementary flow measurement sensorto produce said supplementary flow component at such supplementary flowmeasurement sensor.
 3. Medical apparatus according to claim 1, with saidsupplementary measurement means being operable for supplying asupplementary signal wherein the contribution thereto due to the effectof said parameter at said supplementary flow measurement sensor has amagnitude substantially linearly proportional to the contribution tosaid respiratory flow signal due to the effect of said parameter at saidflow measurement sensor in said breathing tube, the quotient of suchsignals thereby being essentially independent of such contributionssubstantially independently of the gas or gases making up said fluidmedium.
 4. Medical apparatus according according to claim 1, with saidsupplementary flow measurement sensor supplying a continuous analogsignal which is to a substantial degree a predetermined function of theviscosity of the fluid medium, and said evaluation means supplying arespiratory flow indication which is substantially independent of theviscosity of the respiratory gas or gases forming said fluid medium. 5.Medical apparatus according to claim 1, with said supplementary flowmeasurement sensor supplying a continuous analog signal which is to asubstantial degree a predetermined function of the density of the fluidmedium, and said evaluation means being operable for supplying arespiratory flow indication which is substantially independent of thedensity of the respiratory gas or gases forming said fluid medium. 6.Medical apparatus according to claim 1, with said supplementary flowmeasurement sensor supplying a continuous analog signal which is to asubstantial degree a predetermined function of the temperatureconductivity of said fluid medium, and said evaluation means beingoperable for supplying a respiratory flow indication which issubstantially independent of the temperature conductivity of therespiratory gas or gases forming said fluid medium.
 7. In medicalapparatus for respiratory flow measurement, including a breathing tubefor containing a fluid medium subject to respiratory flow varying at abreathing rate, and a flow measurement sensor accommodated in saidbreathing tube for coupling with the respiratory flow of the fluidmedium in said breathing tube to control the supply of a respiratoryflow signal as a function of such respiratory flow, said breathing tubehaving auxiliary means coupled with the fluid medium in said breathingtube for producing a supplementary flow component, supplementary meansresponsive to the supplementary flow component to supply a supplementarysignal which is dependent on a parameter of said fluid medium which alsoaffects the accuracy of said respiratory flow signal, and evaluationmeans operatively connected with said flow measurement sensor andresponsive to said respiratory flow signal and to said supplementarysignal for forming a quotient signal as a function of the quotient ofsaid respiratory flow signal and said supplementary signal to provide arespiratory flow indication compensated for said parameter of the fluidmedium substantially independently of the type of respiratory gas orgases making up said fluid medium, said auxiliary means comprising apump coupled with the fluid medium in said breathing tube for loadingthe respiratory flow so as to produce a supplementary flow componenthaving a frequency which is capable of being differentiated from abreathing frequency of a breathing frequency signal component producedby the flow measurement sensor in response to said respiratory flow atsaid breathing rate, and said supplementary means being connected withsaid flow measurement sensor to receive therefrom a supplementary signalcomponent having the frequency of said supplementary flow component, andbeing operable for separating said breathing frequency signal componentand said supplementary signal component and for supplying asupplementary signal which is a function of said supplementary signalcomponent as a separate input to said evaluation means, and saidevaluation means being operable for supplying a respiratory flowindication which is a function of the quotient of such signal componentsso as to correct the respiratory flow signal in accordance with thecomposition and characteristics of the respiratory gas or gases formingsaid fluid medium.
 8. Medical apparatus in accordance with claim 7, withsaid auxiliary means comprising an alternating pressure pump coupledwith the fluid medium of said breathing tube and operating at afrequency which is capable of being differentiated from the breathingfrequency corresponding to said breathing rate.
 9. Medical apparatusaccording to claim 7, with a patient's mouthpiece for couplingrespiratory flow to the fluid medium of the breathing tube, and flowresistance means providing a flow resistance of substantial magnitudebetween the mouthpiece and the breathing tube.
 10. Medical apparatusaccording to claim 9, with said flow resistance means comprising a hoseproviding a flow path for said fluid medium of substantial length.
 11. Amedical apparatus according to claim 5, with said supplementary meanscomprising electronic filter means connected with said flow measurementsensor and tuned to the frequency of said supplementary flow componentfor separating the supplementary signal component from the breathingfrequency signal component and for controlling the supplying of saidsupplementary signal to said evaluation means, and low-pass filter meanscoupled with said flow measurement sensor to receive therefrom saidbreathing frequency signal component and for controlling the supplyingof said respiratory flow signal to said evaluation means.
 12. Medicalapparatus according to claim 11, with said supplementary meanscomprising band-pass electronic filter means connected with said flowmeasurement sensor and tuned to the frequency of said supplementary flowcomponent for separating said supplementary signal component from saidbreathing frequency signal component, rectifier means connected withsaid band-pass filter means for rectifying said supplementary signalcomponent, and low-pass filter means connected with said rectifier meansfor smoothing the output thereof and supplying an electricalsupplementary signal for use in modifying said respiratory flow signal.13. In medical apparatus for respiratory flow measurement, including abreathing tube for containing a fluid medium subject to respiratory flowvarying at a breathing rate, and a flow measurement sensor accommodatedin said breathing tube for coupling with the respiratory flow of thefluid medium in said breathing tube to control the supply of arespiratory flow signal as a function of such respiratory flow, saidbreathing tube having auxiliary means coupled with the fluid medium insaid breathing tube for producing a supplementary flow component,supplementary means responsive to the supplementary flow component tosupply a supplementary signal which is dependent on a parameter of saidfluid medium which also affects the accuracy of said respiratory flowsignal, and evaluation means operatively connected with said flowmeasurement sensor and responsive to said respiratory flow signal and tosaid supplementary signal for forming a quotient signal as a function ofthe quotient of said respiratory flow signal and said supplementarysignal to provide a respiratory flow indication compensated for saidparameter of the fluid medium substantially independently of the type ofrespiratory gas or gases making up said fluid medium, said auxiliarymeans comprising an alternating pressure pump coupled with the fluidmedium and operable to provide a supplementary flow component in theform of a sinusoidal pulsation with a frequency substantially above thebreathing frequency corresponding to said breathing rate.
 14. Medicalapparatus according to claim 13, with said alternating pressure pumpproducing a sinusoidal pulsation of a frequency between about 5 andabout 15 hertz.